fslot-address

The arguments to this function are like those of fslot-value, but this function
returns a Lisp integer that represents the address of the slot instead
of the value in the slot.

It is like fslot-address-typed, but works
only for :foreign or
:foreign-static-gcobjects. It cannot be open coded
by the compiler. Please see the caution in description of fslot-address-typed
about using the returned address value.